Prior blockage type fluid particle sensors comprise two stainless steel blocks having closely spaced apart planar surfaces forming a fluid gap, typically 1000 microns across, through which the fluid that is to be analyzed flows. Each block has a bore therethrough, the bores having a common longitudinal axis perpendicular to the planar surfaces, and each bore contains a transparent rod having inside and outside ends, with the inside end of each rod being aligned with the planar surface of the stainless steel block that holds the rod, so that the inside ends of the rods are separated by the fluid gap. One of the rods, hereinafter the masked rod, is covered on both ends by a pair of mask plates having identical aligned rectangular apertures. A light source is located adjacent the outside end of the masked rod, and an optical detector system is located adjacent the outside end of the other rod, hereinafter the unmasked rod.
Light rays from the light source enter the masked rod through the aperture in the mask plate covering the outside end of the masked rod. The size of the aperture is typically 150 microns across. The light rays are reflected by the inner walls of the masked rod until they exit through the aperture in the mask plate covering the inside end of the rod. The light rays then cross the fluid gap between the blocks and enter the inside end of the unmasked rod, pass through the rod, and exit the outside end of the unmasked rod, where they are detected by the optical detector system.
Particles in the fluid passing through the fluid gap block some of the light rays, thereby creating a shadow which is sensed by the optical detector system.
A problem with these prior blockage-type fluid particle sensors is that the optical detector system can be fooled. Since the optical detector system sees only shadows and not the actual particles in the fluid, the size of a particle as measured by the optical detector system depends on the size of the shadow cast by the particle, and the number of particles in the fluid as measured by the optical detector system depends on the number of individual shadows cast by the particles in the fluid.
To create a shadow, a particle must be within the view volume of the sensor, i.e., the volume that is illuminated by the light rays passing from the masked rod to the unmasked rod through the gap.
The effective view volume of prior sensors is a truncated pyramid having as its small end the aperture at the inside end of the masked rod, and having as its base the inside end of the unmasked rod. Because of the shape of this effective view volume, a particle near the unmasked rod creates a much larger shadow than a particle near the unmasked rod, so that a particle near the masked rod will be sensed as being much larger than the same particle near the unmasked rod. Sizing errors can be as great as 20:1.
Also, because of the large size of the effective view volume of prior sensors, the maximum number of particles per unit volume of fluid that can accurately be counted is approximately 5,555 particles per millimeter. The larger the view volume, the smaller the number of particles per unit volume of fluid that can be accurately counted. This is because an accurate count, irrespective of the size of the particles, is only achieved when only one particle at a time is present in the view volume. The detector cannot distinguish between the light blockage or shadows of two particles in the view volume at the same time, so it will yield an electrical signal response equal to one apparently larger particle when this condition occurs. Decreasing the size of the view volume decreases the probability of two particles being in the view volume at the same time. Thus, decreasing the size of the view volume increases the number of particles per unit volume of fluid that can be accurately counted.
Of course, the view volume of a prior sensor can be reduced in size simply by reducing the mechanical proportions of the sensor, but this presents the problems of increased operational pressure requirements, reduced sample flow rates, and flow blockage due to plugging of the microscopic sensing zone by the particles to be measured and counted. Furthermore, such a reduction of the proportions of a prior sensor will not change the shape of the view volume, a truncated pyramid, so that sizing errors will still result.
It has been recognized by the inventor of this application that the optimum view volume is a rectangular solid extending across the fluid gap and having a cross section that equals the area of the aperture in the mask plate covering the inside end of the masked rod. This view volume is optimal partly because of its shape. A particle therein casts approximately the same shadow when it is near the unmasked rod as when it is near the masked rod. This view volume is also optimal because it is the smallest obtainable view volume, so that counting errors are minimized.
This optimum view volume can only be obtained if an extremely well collimated beam of light is transmitted through the aperture at the outside end of the masked rod, and this is very difficult, if not impossible, on this small scale (150 microns) with current technology.
Another problem that causes both sizing errors and counting errors is that stray light that reaches the optical detector system will affect particle sensing. Stray light can reach the optical detector system, for instance, by entering the inside end of the unmasked rod or the outside end of the masked rod and being transmitted through the rod or rods to the optical detector system. Stray light entering the rods at almost any angle will eventually be transmitted to the optical detector system.
Another problem with prior sensors is that the apertures at the opposite ends of the masked rod must be perfectly aligned to maximize energy through-put and to minimize the effective view volume.